text: In 2016, researchers from the University of Bristol claimed to have constructed one of those Ni prototypes.
  From their Frequently Asked Questions (FAQ document), the estimated power of a small C-14 cell is 15 J/day for thousands of years. (For reference, a AA battery of the same size has about 10 kJ total, which is equivalent to 15 J/day for just 2 years.) They note it is not possible to directly replace a AA battery with this technology, because a AA battery can produce bursts of much higher power as well. Instead, the diamond battery is aimed at applications where a low discharge rate over a long period of time is required, such as space exploration, medical devices, seabed communications, microelectronics, etc.